I know that the verb pluck means to pull out or pull at, but what's the definition when used as a noun?

Pluck has several meanings as a noun: 1) an act of pulling, or a tug; 2) an animal's heart, liver, lungs, and windpipe, used for food; 3) courage to meet danger or difficulty. Consider this sentence, which includes all three definitions: Debra displayed pluck with her pluck of pluck.

For a literary usage, head to Tolstoy's Anna Karenina. Count Vronsky is passionately fond of horses and racing. He has purchased an English thoroughbred and anticipates winning the officers' steeplechase. Here, Vronsky's English horse trainer comments on what it takes to win a race:

"In a steeplechase it all depends on riding and on pluck," said the Englishman.

Of pluck, . . . Vronsky did not merely feel that he had enough; what was of far more importance, he was firmly convinced that no one in the world could have more of this "pluck" than he had.